This invention relates to a method of forming contours representative of high density video information on a disc substrate. More particularly, this invention relates to a method of recording video information on a disc in a manner suitable for subsequently forming replicated discs for playback on a video player.
A system for recording and playback of video information has been described in a copending application Ser. No. 126,772 of Jon K. Clemens filed Mar. 22, 1971 now Pat. No. 3,842,194. As described in the Clemens application, a lacquered surface is deposited on a thick aluminum disc and a continuous spiral groove is cut into the lacquered surface. A first nickel replica is then made of the grooved lacquer surface by depositing nickel over such surface and thereafter separating the nickel coating from the lacquer. This first nickel replica is a negative reproduction of the original grooved lacquer. A second nickel replica is made of the first replica to form a metal disc that is a positive reproduction of the original grooved lacquer. A relatively uniform coating of electron beam sensitive material, such as a photoresist, is thereafter formed on the second nickel replica.
The photoresist-coated replica is then exposed by a video-signal-modulated beam of a scanning electron microscope which provides exposure of the photoresist in the groove of the disc in correspondence to the video signal information. The photoresist is then developed and the exposed portions of photoresist are removed to form a topography in the groove corresponding to the video signal information. A nickel replication is made of the resultant disc and this replication is utilized to stamp or emboss vinyl records by techniques known in the audio recording art. The vinyl replica is then metalized to make the surface conducting and the metalization is thereafter coated with a dielectric. In playing back the recorded information, a stylus is caused to ride in the dielectric coated groove. This stylus, along with the metalization and dielectric, acts as a capacitor. Capacitance variations in the groove, which correspond to the recorded video information, are then detected electronically to recover the video information.
In the system of the above-described Clemens application, if the photoresist-coated replica is formed with a thin layer of photoresist, that is, the layer is of such thickness as to allow the electron beam to completely pierce the photoresist layer and impinge upon the substrate, subsequent exposure of this photoresist coating by the electron beam can provide a non-uniform topography. A non-uniform topography is formed when exposure of the electron beam sensitive material (photoresist) is inadvertently varied by, for example, uncontrolled reflection of the impinging electron beam off the substrate material. In the exposure of a thin layer of electron beam sensitive material, the substrate upon which this thin material layer is formed is used as a reference plane; that is, the electron beam exposure is adjusted to provide penetration through the electron beam sensitive material to the substrate. When the substrate material is utilized as a reference plane, the depth of the formed topography varies as a function of thickness of the electron beam sensitive material. Since it is difficult to apply a uniformly thin material coating over a grooved substrate, it is difficult to provide a topography having exposed regions formed to a uniform depth. During exposure of the thin photoresist material, the photoresist-piercing electron beam reflects off of the metal substrate. In addition to the exposure by the incident beam, the reflected electrons also expose portions of the photoresist material, increasing the width of the electron-beam inscribed area in accordance with the quantity of reflected electrons. This quantity of reflected electrons varies in accordance with the number of electrons striking the substrate and is related to the material coating thickness at the point of exposure. Electron beam reflection off the substrate material at positions where the photoresist coating is too thin, may overexpose the area causing undercut (pear-shaped) exposures in the photoresist material. Undercut or pear-shaped exposures in the photoresist material make the replication process difficult in that the replicating metal coating that is formed over the exposed photoresist becomes extremely difficult to separate from the photoresist-coated disc without fracturing or otherwise deforming the thin metal coating.